An XY-wing is three cells with two candidates each, arranged as a pivot that sees two pincers. The pivot holds X and Y; one pincer shares its X and adds a third digit Z, the other shares its Y and adds the same Z. The pincers need not see each other — only the pivot must see both.
Whichever way the pivot resolves, a pincer becomes Z. If the pivot takes X, the {X,Z} pincer loses its X and lands on Z; if it takes Y, the {Y,Z} pincer does. So Z is guaranteed to sit in one pincer or the other — and any cell that sees both pincers can never hold Z.
To spot one, start from bivalue cells: find a cell whose two candidates each reappear in another bivalue cell it sees, with those two cells agreeing on a shared third digit. The pivot is the hinge; the eliminations land where the pincers’ lines of sight overlap.
On the board
The pivot here is R4C9, holding {3,6}. Along row 4 it sees the pincer R4C4 {1,6}; through the right-middle box it sees the pincer R6C7 {1,3}. If R4C9 is 6, then R4C4 cannot be and must take its 1; if R4C9 is 3, then R6C7 must take its 1 instead. Either way a 1 lands on a pincer — so the two cells that see both pincers lose theirs: R4C7 (row 4 with one, the box with the other) and R6C4 (column 4 with one, row 6 with the other).
This is the exact board the app's interactive XY-Wing demo uses — step 44 of its 70-step walkthrough, drawn here with its live pencil marks.
